A concept a lot of us are probably much more familiar with now thanks to lockdown life, the Zoom effect, and perhaps juggling home-schooling as well. Even simple tasks such as going to the shop require the extra concentration of remembering to wear a mask and keep physically distant from others. Those who are working from home may be finding it more difficult, for example, missing out on useful information that gets shared through conversation rather than in a meeting which they would hear about in an office setting.
One of the benefits of running and other exercise, alongside all the physical health gains, is that it can help us to 'switch off' mentally and be a release from the stresses of the working day. Interestingly, however, mental fatigue has been found to negatively affect physical performance. A study had the participants either engage 90 minutes of a cognitively demanding task or watch an emotionally neutral documentary then cycle to exhaustion at 80% of maximum. They found that following the cognitively demanding task the time to exhaustion was significantly reduced (e.g. 10 minutes rather than 12 minutes) and that perceived exertion was higher. There appeared to be no physiological factors that would account for the difference in performance between the two conditions and it was concluded that exercise tolerance can be limited by mental fatigue. Other research suggests that endurance performance is more affected than maximal anaerobic performance (e.g. short duration, high intensity exercise).
Having worked the desk based 9-5 environments in a previous life I'm aware how much of a struggle it can be to get moving particularly at the end of a mentally challenging day. I think it is really reassuring to know that this type of fatigue is real and has a real effect.
Use this information to be strategic about how you schedule longer steady state workouts such as running or cycling. This suggests that it would be sensible to plan any longer runs at times when you are in a mentally well rested state perhaps in the mornings or at the weekends. Especially if you are in training for an endurance event (10km or above) and working to increase distance. If arranging workouts that way isn't possible it may be beneficial to spend some time on a non-cognitively demanding task before starting the workout. For example, using a meditation app for 5-10 minutes, reading a book, or even starting out at a walking pace for the first few minutes until you feel refreshed then picking up the pace. As performance in high intensity workouts is less likely to be affected by mental fatigue those could fit in even at the end of a tough day.